The general principle of transmitting data by dividing it into several interleaved bit streams, and using these bit streams to modulate several carriers is well known, e.g., Discrete Multitone (DMT) and Orthogonal Frequency Division Multiplex (OFDM) modulation and demodulation systems. These types of multi-carrier modems are being used or considered for use in such applications as cellular radio and Digital Subscriber Lines (DSLs) such as High rate Digital Subscriber Lines (HDSLs), Asymmetric Digital Subscriber Lines (ADSLs), etc.
In a Discrete Multitone system, the input bit stream is first serial-to-parallel converted. The parallel output is then grouped into N groups of bits corresponding to the number of bits per symbol. Portions of bits are allocated to each DMT carrier or subchannel. The power transmitted over each subchannel is preferably approximately the same.
FIG. 1 shows an example Discrete Multitone (DMT) communication system in which the present invention may be advantageously employed. Transmitter 10 includes a serial-to-parallel converter 14, a multicarrier modulator 16, and a pretransmit processor 18. Receiver 12 includes a post channel processor 20, a multicarrier demodulator 22, and a parallel-to-serial converter 24. The transmitter and receiver are linked in this example by a digital subscriber line (DSL) or other form of communication channel 26. Serial input data at a rate of b.sub.total /T bits per second are grouped by converter 14 into blocks of b.sub.total bits for each multicarrier symbol, with a symbol period of T. The b.sub.total bits in each multicarrier symbol are used to modulate N separate carriers in modulator 16 with b.sub.i bits modulating the i.sup.-th carrier.
A preferred embodiment uses an Inverse Discrete Fourier Transform (IDFT) during modulation to generate N.sub.s time-domain samples of a transmit signal for each block of b.sub.total bits, where N.sub.s is preferably equal to 2N. The corresponding multicarrier demodulator performs a Discrete Fourier Transform (DFT), where b.sub.i bits are recovered from the i.sup.-th carrier. As depicted in FIG. 2, the carriers or subchannels in a DMT system are spaced 1/T Hz apart across N/T Hz of the frequency band. More detailed discussion of the principles of multicarrier transmission and reception in general is given by J. A. C. Bingham in "Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come", IEEE Communications Magazine, Volume 28, Number 5, pp. 5-14, May 1990.
In some digital subscriber link systems as well as in different radio systems where multi-carrier modulation is used, the modulation output is approximately a normal distribution. Normal distribution means that the peak-to-average ratio of the output is relatively high. Because of this high ratio, the transmitting amplifier in a multi-carrier system, (e.g., a line driver in a digital subscriber line system, a power amplifier in a radio system, etc.), must have a high supply voltage in order to adequately transmit the occasional high signal peaks without "clipping" or other distortion.
Unfortunately, such a high supply voltage results in substantial power dissipation in the line driver, power amplifier, etc. In fact, in a typical commercial, Asymmetric Digital Subscriber Line (ADSL) system, more than sixty percent of the total power is consumed in the line driver. Thus, there is a need to reduce the power dissipation in such a line driver as well as in other multicarrier modulator drivers.
To illustrate the power dissipation in a line driver, consider the DMT Digital Subscriber Line (DSL) example shown in the simplified diagram FIG. 3. A modulated multi-carrier signal is converted into analog format in a digital-to-analog converter 30 and then supplied to a line driver amplifier circuit 32 which drives the "line" 34 via a coupling transformer. A schematic representation of FIG. 3 is shown in FIG. 4 with the modulated input signal being represented as U.sub.in. The amplified output signal U.sub.L produced by line driver 32 is supplied to a resistive load (R.sub.L) 38. An amplifier voltage supply 34 includes positive and negative voltage supply "rails," i.e., the supply rails have the same magnitude but opposite polarity.
The power dissipated in the line driver 32 (P.sub.d) may be characterized in accordance with the following equation: EQU P.sub.d =(P.sub.SUPPLY -U.sub.L)*U.sub.L /R.sub.L +P.sub.f (1)
The parameter P.sub.f is a technology dependent power that can only be reduced with new possible improvements in semiconductor technology fabrication/manufacture. Therefore, treating P.sub.f as a constant (or at least something that cannot be directly controlled), the dissipated power line driver P.sub.d may only be reduced by lowering the supply voltage V.sub.supply.
The graph in FIG. 5 shows an example output signal from a DMT or OFDM modulation transmitter having a normal distribution. The "tails" of the normal distribution curve are quite long and correspond to a relatively high peak voltage V.sub.high. Even though most of the "tail" indicates a very low probability of occurrence, in order to accurately generate the infrequent high magnitude voltages without clipping, the line driver requires a relatively high supply voltage.
It would be highly desirable to selectively supply a high magnitude voltage V.sub.high to a multi-carrier, transmit/driving device, like a line driver, for high magnitude input signals and supply a considerably lower magnitude voltage V.sub.low when the input signal magnitude is relatively low. If lower supply voltages were used most of the time, the overall power dissipation in the driving device would be considerably reduced.
The present invention achieves the desirable result of reduced power dissipation. In particular, an amplifier circuit receives an input signal and generates an output signal. First and second power supplies provide power at first and second levels, respectively, where the second level is greater than the first level. A controller causes power to be supplied from the first power supply to the amplifier circuit when the magnitude of the input signal is less than or equal to a predetermined threshold. When the magnitude of the input signal is greater than the threshold, the controller causes power to be supplied from the second power supply to the amplifier circuit.
In preferred example implementations of the invention, the amplifier circuit is a line driver used in a multi-carrier, DSL-type transmission system. The first power level preferably corresponds to a voltage of five volts, and the second power level preferably corresponds to a voltage of twelve volts. The controller includes a comparator which compares an amplitude of the input signal with the threshold and generates the control signal based on the comparison.
In one example embodiment of the present invention, the amplifier circuit includes a single amplifier having a power input. A switch is coupled to the first and second power supplies and to the power input of the amplifier circuit. The controller generates a control signal that controls the switch to selectively couple either of the first and second power supplies to the power input of the amplifier.
In another example embodiment, the amplifier circuit includes first and second amplifiers coupled to the input signal. The first amplifier is coupled to the first power supply and the second amplifier is coupled to the second power supply. A switch is connected to an output from each of the first and second amplifiers. The controller generates a control signal causing the switch to select the output from either of the first and second amplifiers.
Accordingly, the present invention provides a method for reducing the power dissipated by an amplifier circuit. In particular, the power supplied to the amplifier circuit is changed depending on the level of input signal to that circuit. A higher power is supplied to the amplifier circuit when the level of the input signal exceeds a threshold. Otherwise, a lower power is supplied to the amplifier circuit. The magnitude of the input signal is detected and compared with the threshold. A control signal is generated based on that comparison to control whether higher or lower power is supplied to the amplifier circuit.